(a) Field of the Invention
This invention relates to automatic transmissions that provide six forward speeds by means of three clutches and two brakes.
(b) Description of the Related Art
As an automatic transmission of this type, there is conventionally known one that includes an input shaft, a speed reducing gear set for reducing the rotation speed of the input shaft, a Simpson planetary gear train formed by combining two simple planetary gear sets, three clutches, two brakes and an output shaft and is configured to appropriately selectively engage and release the three clutches and two brakes serving as friction elements (see, for example, FIG. 1 in Unexamined Japanese Patent Publication No. H04-219553 (hereinafter, referred to as Patent Document 1)).
In the above automatic transmission, in order to provide an overdrive gear, inputs to a carrier and an associated ring gear of the planetary gear train are necessary. If, however, the input shaft and the output shaft are arranged on the same axis, it is impossible to establish the input path connecting to both the carrier and the ring gear of each simple planetary gear set having only three rotation elements. Therefore, in the above automatic transmission, no alternative exists but to arrange the input and output shafts in parallel on different axes, which leads to upsizing the transmission.
On the other hand, for example, FIGS. 3 and 13 in the same document discloses a technique using, instead of the Simpson planetary gear train, a so-called Ravigneaux planetary gear train (a compound planetary gear train of double pinion type in which two sun gears are meshed with each pair of inner and outer pinions, respectively). Further, in this technique, in order to arrange the input and output shafts on the same axis, a single pinion type planetary gear set is used as a speed reducing gear set for reducing the speed of input rotation.
However, generally, the Ravigneaux planetary gear train has a problem that its gear noise and vibrations are likely to become large because of its complex structure and a large number of meshing gears. Further, the Ravigneaux planetary gear train has various problems of higher cost than the Simpson planetary gear train, difficulty in ensuring its strength and production of torque circulation inside the planetary gear train.
In view of the above points, as disclosed in, for example, the pamphlet of International Patent Publication No. WO02/099316 (hereinafter, referred to as Patent Document 2) and Unexamined Japanese Patent Publication No. 2004-069050 (hereinafter, referred to as Patent Document 3), there is proposed a technique in which a Simpson planetary gear train, not a Ravigneaux planetary gear train, is employed and one of two planetary gear sets forming the gear train is composed of a so-called double ring gear type or double sun gear type planetary gear set to allow its center member to radially input and output the rotation into and out of the carrier.
—Problems that the Invention is to Solved —
The double ring gear type or double sun gear type planetary gear set as in the proposed technique, however, cannot be obtained by simply separating a ring gear or a sun gear in halves but must be configured so that individual separated ring or sun gears have sufficient strength in consideration of their power transmission.
Therefore, the above planetary gear set such as a double ring gear type planetary gear set has a profile of approximately the same size as a combination of two planetary gear sets, which is a substantially large in the direction of the rotation axis. Further, the pinion bridged over and meshed with the separated ring or sun gears is long and, therefore, is likely to tilt during rotation. Hence, the above planetary gear set tends to produce large gear noise and vibrations, not as large as the Ravigneaux planetary gear train.
The present invention has been made in view of the foregoing points and, therefore, its object is to make compact a six-speed automatic transmission of a type as proposed above while restraining production of gear noise.